The micellar structures formed by the perfluorooctylsulfonate ion (FOS -) in combination with the tetraethylammonium (TEA + ) and lithium (Li + ) counterions are studied by measuring the NMR chemical shifts and self-diffusion coefficients of both TEA + and FOS -at 30°C. Spherical micelles generated by LiFOS are characterized by a lifetime of 10 -6 s, an aggregation number of about 36, and a cmc value of 7.16 mM. The fluorine spectrum of the FOS -ion in the micellar solutions of LiFOS consists of one set of peaks whose chemical shifts are averaged between the monomer and micellar states. When combined with the hydrophobic TEA + counterion, the FOS -ions show the 19 F spectra which consist of two sets of peaks whose chemical shifts and intensities are concentration dependent. The 19 F spectroscopic features are discussed by assuming the appearance of small aggregates at high surfactant concentrations. The binding coefficient (0.80) of the TEA + counterions to FOS -micelles is determined on the basis of the self-diffusion coefficients, measured for both TEA + and FOS -. The micellar self-diffusion coefficient in the TEAFOS system (8.4 × 10 -12 m 2 s -1 ) is found to be 1 order of magnitude lower than that in LiFOS system (5.7 × 10 -11 m 2 s -1 ), reflecting the dynamic slowdown due to the formation of threadlike micelles.

We studied the pressure-induced folding/unfolding transition of staphylococcal nuclease (SN) over a pressure range of approximately 1-3 kilobars at 25 degrees C by small-angle neutron scattering and molecular dynamics simulations. We find that applying pressure leads to a twofold increase in the radius of gyration derived from the small-angle neutron scattering spectra, and P(r), the pair distance distribution function, broadens and shows a transition from a unimodal to a bimodal distribution as the protein unfolds. The results indicate that the globular structure of SN is retained across the folding/unfolding transition although this structure is less compact and elongated relative to the native structure. Pressure-induced unfolding is initiated in the molecular dynamics simulations by inserting water molecules into the protein interior and applying pressure. The P(r) calculated from these simulations likewise broadens and shows a similar unimodal-to-bimodal transition with increasing pressure. The simulations also reveal that the bimodal P(r) for the pressure-unfolded state arises as the protein expands and forms two subdomains that effectively diffuse apart during initial stages of unfolding. Hydrophobic contact maps derived from the simulations show that water insertions into the protein interior and the application of pressure together destabilize hydrophobic contacts between these two subdomains. The findings support a mechanism for the pressure-induced unfolding of SN in which water penetration into the hydrophobic core plays a central role.

We have used neutron spin echo (NSE) spectroscopy to study the effect of bilayer thickness and monounsaturation (existence of a single double bond on one of the aliphatic chains) on the physical properties of unilamellar vesicles. The bending elasticity of saturated and monounsaturated phospholipid bilayers made of phospholipids with alkyl chain length ranging from 14 to 20 carbons was investigated. The bending elasticity κ(c) of phosphatidylcholines (PCs) in the liquid crystalline (L(α)) phase ranges from 0.38 × 10(-19) J for 1,2-dimyristoyl-sn-glycero-3-phosphocholine to 0.64 × 10(-19) J for 1,2-dieicosenoyl-sn-glycero-3-phosphocholine. It was confirmed that, contrary to the strong effect on the main transition temperature, the monounsaturation has a limited influence on the bending elasticity of lipid bilayers. In addition, when the area modulus K(A) varies little with chain unsaturation or length, the elastic ratios (κ(c)/K(A))(1/2) of saturated and monounsaturated phospholipid bilayers varies linearly with lipid hydrophobic thickness d which agrees well with the theory of ideal fluid membranes.

At low ionic strength, organic counterions dress a flexible charged polymer as measured directly by small-angle neutron scattering and neutron spin-echo spectroscopy. This dressed state, quantified by the concentration dependence of the static correlation length, illustrates the polymer-counterion coupled nature on the nanometer length scale. The counterions, made visible by selective hydrogen and deuterium labeling, undress from the polymeric template by addition of sodium chloride. The addition of this electrolyte leads to two effects: increased Debye electrostatic screening and decoupled organic counterion-polymer correlations. Neutron spin-echo spectroscopy measures a slowing down of the effective diffusion coefficient of the labeled counterions at the length scale of 8 nm, the static correlation length, indicating the nanosecond counterion dynamics mimics the polymer. These experiments, performed with semidilute solutions of tetramethylammonium poly(styrene sulfonate) [(h-TMA(+)) d-PSS], apply to relevant biopolymers including single and double stranded DNA and unfolded proteins, which undergo orchestrated dynamics of counterions and chain segments to fold, unfold, and assemble.

Mixtures of tetraethylammonium perfluorooctylsulfonate (TEAFOS) and lithium perfluorooctylsulfonate (LiFOS) in water (D 2 O) are studied as a function of the LiFOS fraction (φ Li ) at a total concentration of 100 mM and 30 °C by means of 1 H and 19 F NMR and viscosity measurements. The counterion binding in the double layer structure of the FOS micelles is analyzed through the chemical shifts and self-diffusion coefficients that are sensitive to the Stern and diffuse double layers, respectively. At φ Li ) 0, the fraction of bound tetraethylammonium counterions (TEA + ) due to the proton chemical shift is found to be 0.45; it implies that one TEA + counterion is bound to and bridging roughly two micellized FOSions. This value is markedly smaller than that (0.73) obtained by the diffusion data because of the short-range sensitivity of the chemical shift. The binding fraction due to the diffusion data is higher because it involves both the Stern and diffuse double layers. The concentration of the TEA + counterions preferentially localized within the Stern layer remains constant at 45 mM when φ Li is varied between 0 and 0.55. It is shown that the preferential saturation of the Stern layer with TEA + counterions in this region of φ Li is prerequisite for the formation of the threadlike FOS structure and the high solution viscosity. At higher values of φ Li , the threadlike structure disintegrates and the viscosity drops as a result of an overall shortage of TEA + counterions in the solution.

We apply small-angle neutron scattering (SANS) to study the effect of pressure on micelle structure in a solution of 1% by weight pentaethylene glycol mono-n-dodecyl ether (C12E5) in D2O at 20 °C and pressures up to ∼3000 bar. At ambient pressure, the structure is a network of branched, semiflexible, cylindrical micelles with the branch points comprised of three-armed junctions. Our SANS results reveal that pressure induces a phase transition from this network of threadlike micelles to hexagonally ordered bundles of cylindrical micelles. Using geometric packing constraints for three-arm junctions and cylinders, we show that the formation of three-arm junctions becomes increasingly unfavorable with increasing pressure due to the compression of the micelle hydrophobic core, and as such, the network becomes unstable at pressures close to those observed in our SANS experiments. We also measured the temperature dependence of the transition pressure and find that it follows the pressure-temperature freezing curves for liquid n-alkanes of comparable hydrocarbon chain length. These observations lead us to propose that the phase transition is related to a loss of flexibility or conformational entropy of the C12E5 micelles upon the pressure-induced freezing of the micelle hydrophobic core to form an amorphous solid. The formation of hexagonally ordered bundles of cylindrical micelles follows as attractive van der Waals forces between the micelles are not offset by the loss of repulsive undulation forces arising from the fluidity of the hydrophobic core.

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